This invention relates to a small cartridge or probe type heater, e.g. to a tubular sheathed heating element of small diameter, having a particular application to spacecraft thrusters.
An interesting tubular sheathed heating element of small diameter is shown in U.S. Pat. No. 2,898,571 to Moule et al, for application to diesel glow plugs for compression ignition engines. It has a helical element placed in a magnesium tube, which in turn is placed in a sheath. The element is heat treated to convert the magnesium to magnesium oxide insulation.
Catalyst bed heaters are extensively employed on operational spacecraft monopropellant propulsion subsystems. Probe type heaters for catalyst bed heaters have had various problems, such as element resistance shift, insulation resistance drop, shorting of lead wires, fatigue stress at element to lead wire weld joints, and others.
The function of the catalyst bed heater is to maintain the bed at a temperature which provides improved consistent performance for each pulse. Although the catalyst bed heater is not mandatory requirement for a monopropellant catalytic thruster (because the thruster will function without it), it serves to provide predictable pulses, impulse repeatability and increase catalyst bed lifetime. An important function of the catalyst bed heater is to prevent catalyst bed degradation from cold starts which promotes bed breakup. Therefore, the basic requirement for this type heater is to maintain the bed at a minimum predetermined temperature (usually greater than 200.degree. F.) under worst case conditions.
The prior art include a reflector type space heater in which the heating unit at the center has a truncated-cone-shaped ceramic core with a groove in the form of a single lead thread. The wire element is a coil which has a small helical diameter compared with the diameter of the core at its small end. The coil is laid in the groove of the core, with a straight uncoiled end of the wire returned through the inside of the core. The depth of the groove is only about half the coil diameter.